Smooth Volume Rendering without Denoise in Cycles
There are two settings that affect this - one is the 'Step Rate' that you have found, the other is the Max Steps - and both have an impact.
It used to be that the two settings behaved individually - and the volume would be rendered with a 'Step Size' and then if the Max Steps was reached the volume would simply be ignored for the rest of that light ray. This was fine in most cases but could produce unexpected results, with the volume effectively &quot;cut off&quot; as soon as the limit was reached. Furthermore, changing the Step Size while leaving the Max Steps unchanged would change how much of the volume was rendered.
Now the Step Size has been replaced with the Step Rate and the Max Steps has been enhanced so as not to cut off the render by automatically generating a 'Step Size' behind the scenes based on both the Max Steps and the Step Rate. This means that reducing the Max Steps will no longer cutoff the volume but will affect the size of the steps - so having Max Steps set too low will limit the effect of more precise Step Rate.
The solution is to increase Max Steps to allow the Step Rate to have more of an effect. This will increase render times since more steps are required to traverse the volume but can produce more detail in your volumetrics.